V(X1)=1 and V(X2)=4

The constraint since E(X1)=z=E(X2) is a+b=1, where z should be lower case, not upper case.

So use calc 1 to minimize given a+b=1, by inserting one of these two variables,a,b.

You could use LaGrange, but cal 1 is easier.

I get a=4/5, b=1/5 giving me 20/25=.8 as the answer.